In the processing of integrated circuits, electrical contact is typically made to isolated active device regions formed within a wafer substrate typically comprising monocrystalline silicon. The active regions are typically connected by electrically conductive paths or lines which are fabricated above an insulative material formed over the substrate surface. Further, electrical contact is also typically made to other conductive regions received outwardly of the wafer, such as to conductive lines, contact plugs and other devices. To provide electrical connection between two conductive regions, an opening in an insulative layer is typically etched to the desired regions to enable subsequently formed conductive films to make electrical connection with such regions.
The drive for integrated circuits of greater complexity, performance and reduced size has driven designers to shrink the size of devices in the horizontal plane. Yet to avoid excessive current density, the horizontal scaling has not necessarily been accompanied by a reduction in the vertical dimension. This has resulted in an increase of the ratio of device height to device width, something generally referred to as aspect ratio, and particularly with respect to contact openings. Such currently ranges from 1.0 to 5, and is expected to increase. The circuit density increase places increasing constraints on the conductivity of the contacts themselves.
As transistor active area and other device dimensions approached 1 micron, conventional process parameters resulted in intolerable increased resistance between the active region or device area and the conductive layer. A principal way of reducing such contact resistance is by formation of a metal silicide atop the active area prior to application of the conductive film for formation of the conductive runner. Common metal silicides are refractory metal suicides, such as TiSix, where “x” is predominately 2. The TiSix material is typically provided by first applying a thin layer of titanium atop the wafer which contacts the silicon containing active areas within the contact openings. Thereafter, the wafer is subjected to a high temperature anneal. This causes the titanium to react with the silicon of the active area, thus forming the TiSix. Such a process is said to be self-aligning, as the TiSix is only formed where the titanium metal contacts silicon. The applied titanium film typically everywhere else overlies an insulative, and substantially non-reactive, SiO2 layer. After the first annealing, unreacted titanium may be removed selectively relative to the formed silicide by a wet etch. Further, a post-silicidation anneal might be conducted to lower sheet resistance of the formed silicide.
In the silicidation process, silicon from contact regions of the substrate diffuses upward into the refractory metal layer. Similarly, the refractory metal diffuses into the underlying silicon. The intent is for the titanium and silicon to react with each other to form a silicide thick enough to provide low sheet resistance and make a highly conductive contact interface. As a result, the doped active area of the silicon substrate (or other silicon construction) becomes thinner due to the consumption of silicon during the reaction. The resultant silicide is said to intrude or sink into the substrate or device. Over-consumption of the underlying silicon can be problematic for any silicon circuit element, tending to cause voids and thus device failures. Tendency in the industry is to make shallower and shallower active area junctions in the silicon substrates. In some instances, silicide contacts of sufficient thickness cannot be formed without completely destroying a junction because of silicon consumption from the underlying substrate.
The invention was principally motivated in addressing these problems, but is not so limited and has other applicabilities as will be appreciated by the artisan.